The present invention relates to a safety selector switch for preventing accidental energization of an electrical circuit by requiring a very definite sequence of operations in order to rotate an electrical device, such as a rotary switch, to its various positions.
Control panels, such as those used in aircraft, often have large numbers of electrical components such as switches and potentiometers, and because of space limitations these components are often mounted closely together. Consequently, it is very likely that a component may be unintentionally rotated when it should not be, which could create an unsafe condition.
Various locking devices have been provided to prevent accidental rotation of a rotary device. One well-known device is described in U.S. Pat. No. 2,917,942, "Rotary Locking Device", which issued Dec. 22, 1959, to John W. Jarrett and Everett L. Rice. This patented device comprises a shaft that is axially slidably attached to the existing shaft of an electrical component, and the attached shaft has at least one lug that is engageable with a plurality of slots in a shaft bushing. The attached shaft is resiliently biased outwardly to retain the lugs within one of the slots and when in this position, the attached shaft is locked from rotation. When it is desired to rotate the attached shaft, it is first necessary to move it inwardly until the lug is disengaged from the slot. The shaft is then free to be rotated. However, the disadvantage of this patented device resides in the fact that once the attached shaft is moved inwardly, then the shaft is free to be rotated to any angular position and thus the shaft can be accidentally rotated to a position that would create an unsafe condition. For example, in a bomb arming system it may be necessary to turn a shaft to several warm-up positions, or test positions, before turning to an arming position and no heretofore known device can provide the necessary safety function of permitting rotation to a plurality of positions and providing locking means for each position.
Another safety locking device for a rotary switch is described in U.S. Pat. No. 3,139,769, entitled "Rotary Locking Device", which issued July 7, 1964, to Richard H. Gauchat. In this patented device, a housing is attached to the threaded portion of a rotary switch and a shaft carried by the housing is axially slidably attached to the existing shaft of a rotary switch. A pair of locking pins are provided on the attached shaft and these pins are engageable with a locking plate which limits the amount of rotational travel of the attached shaft. In order to obtain additional rotation, the pins must be shifted axially, that is, an alternating push-pull motion is required in order to turn the attached shaft to its various positions. Thus, accidental rotation can be avoided, as a very definite sequence of operation is required in order to turn a rotary switch to its various positions.
In the above-identified patent application of James D. Hansen, Jr., there is disclosed a safety selector switch which requires alternating rotation of a first shaft and translation of a second shaft for operation and, upon reaching a terminal position, first and second light sources emit light to first and second light detectors to close an energizing circuit. Light from the first light source can pass through an aperture in a disc only after the disc has been rotated to a predetermined position. The light source which provided light that passed through an aperture in a disc is a relatively large distance from the light detector and, during testing, it was determined that the switch would not be operable at a temperature above 56 degrees C.